Method of mulching with antistatic synthetic polymer film

ABSTRACT

A method of mulching comprising mulching plants or soil with antistatic films of a synthetic polymer obtained by the method consisting essentially of continuously printing over at least part of the surface of at least one side of the film of synthetic polymer with aluminum powder by suspending in solvent a composition consisting essentially of aluminum powder and a polyamide resin, setting the suspension down onto the film, and permitting the solvent to evaporate. The films obtained have a surface resistivity of less than 10 10  ohms/cm. The mulching of plants with these films prevents virus degeneration of the plants by virus-carrying aphides, and the mulching of the soil limits the increase of soil temperature in warm environments.

This is a division of application Ser. No. 748,214, filed Dec. 7, 1976,now U.S. Pat. No. 4,221,830.

BACKGROUND OF THE INVENTION

The present invention relates to a method of mulching using antistaticfilms of synthetic polymers.

It is known that synthetic polymers, owing to their non-conductivity,readily acquire but do not easily lose static electrical charges. Thegeneration of static electricity onto the surface of plastic materialsmay occur in several ways during the period extending from theirproduction to their final use. One way is through the friction caused byextrusion or even through pulling a film over the rollers of a packagingmachine.

Static electricity has detrimental effects in various applications ofsynthetic polymers. In the packaging industry, first it causes theattraction of dust which ruins the clarity of a polymer film, secondly,it causes handling problems which result from the polymers adhering tometal parts of the packaging machine, and thirdly it causes electricaldischarges which may start a fire and/or explosion.

Previously, two means have been proposed for resolving these problems.The first means is the use of air-ionizing bars which provide negativeand positive ions that neutralize the plastic. This is an effectivemeans of reducing fire and explosion hazards, but provides onlytemporary protection. The second means is the incorporation into thesynthetic polymer of antistatic chemical agents such as quaternaryammonium compounds, phosphate esters, and polyglycol esters of fattyacids which are hygroscopic and remain on the polymer surface.Nevertheless, depending on the method of incorporation used, they havethe disadvantage of being easily removed from the plastic surface byhandling or cleaning or of only producing their effect for several daysor several weeks after incorporation. Furthermore, surface resistivityof polymers comprising such agents cannot be reduced below 10¹⁰ ohms/cm,whereas surface resistivity of unmodified polymers is about 10¹⁶ to 10¹⁸ohms/cm.

SUMMARY OF THE INVENTION

The object of this invention is to develop uses for antistatic films ofsynthetic polymers in agricultural applications.

Additional advantages will become apparent from the followingdescription, or may be deduced from the practice of the invention. Toachieve this object, the invention provides for mulching plants or soilwith a film of an antistatic polymer. The film is made by continuouslyprinting over at least a part of the surface of a film of syntheticpolymer with a conducting powder, wherein the synthetic polymer isselected from low density polyethylene, high density polyethylene,polypropylene, and polybutene-1 and wherein the conducting powder isaluminum powder. More precisely, the method consists essentially ofcontinuously printing over at least part of the surface of at least oneside of the polymer film with aluminum powder by suspending in solvent acomposition consisting essentially of aluminum powder and a polyamideresin, setting the suspension down onto the film, and permitting thesolvent to evaporate. The solvent is constituted by a mixture ofisopropyl alcohol, petrol, and possibly dry butanol.

The expression "the surface of a film" means the surface of at least oneside of a film.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description is intended to make the invention easier tounderstand, but it should not be interpreted as limiting it.

The synthetic polymers which may be used in the method according to theinvention are particularly preferred over non-olefinic polymers such aspolyvinyl-chloride and cellophane, especially in view of the surfaceresistivity that can be achieved.

The method of making the film according to the invention may useflexographic printing as well as with heliographic printing, so-calledtransfer helio or direct helio. The choice of the printing method willdepend on the width of the film and on the consumption of ink comprisingaluminum powder which may be allowed for a given cost. Aluminum powderof low mean grain size is preferred.

The method of making the film described above is particularlyadvantageous both from the practical as well as the economical point ofview since it uses well known techniques and facilitates a reduction inthe cost of the films without loss of quality compared to methods usingionization of air or the incorporation of chemical agents.

The electrical properties of the film obtained according to theinvention are better than those of antistatic films obtained byconventional methods. Thus, the resistivity may be reduced to 10⁸ohms/cm and possibly to even lower values. On the other hand, theseelectrical properties are achieved at once and are maintainedsatisfactorily in the course of time.

Tests in some agricultural applications have unexpectedly shown that theantistatic films possess remarkable properties under certain conditionswhich make them useful in the field of agriculture. Thus, it is a wellknown problem for cultivators to exterminate aphides which act asvehicles of the virus degeneration of some plants. It has been foundthat by covering plants exposed to that phenomenon with antistatic filmsaccording to the invention, there is achieved an important increase oftheir output due to a powerful repulsive effect towards aphides. Thesefilms can therefore be beneficially used for the mulching of late autumncultures such as tomatoes, pimentoes, marrows, etc.

In another field, moreover, it has been found that mulching withantistatic films according to the invention shows specific advantages incountries with warm environments in order to limit the increase in soiltemperature.

To illustrate the production of the antistatic film used in theinvention more concretely, reference is made to the following example,which should not be taken as limiting the scope and general principlesof the invention.

EXAMPLE

A solvent mixture is prepared of by weight 63% isopropyl alcohol, 32%petrol, and 5% dry 1-butanol. A composition comprising 80% by weightpolyamide resin and 20% by weight of an aluminum powder having a meangrain size of 6 microns, a covering power of 30,000 cm² /g, and whereof0.1% of the grains have a diameter greater than 45 microns is put intosuspension in this solvent mixture. The resulting suspension is set downonto a film of low density polyethylene having a thickness of 50 micronsand a width of 1.20 meters by the flexographic printing method. Then thesolvent mixture is evaporated.

The resistivity of the film is measured in conventional manner byapplying a known voltage to the latter and reading off the value of theintensity of the current. The surface resistivity of the sample thusprepared by the method of the invention is 10⁸ ohms/cm.

What is claimed is:
 1. A method for preventing the virus degeneration ofplants by virus-carrying aphides comprising mulching plants exposed tosuch degeneration with antistatic films obtained by the methodconsisting essentially of continuously printing over at least part ofthe surface of at least one side of a film of synthetic polymer withaluminum powder by suspending in solvent a composition consistingessentially of aluminum powder and a polyamide resin, setting saidsuspension down onto said film, and permitting said solvent toevaporate, said synthetic polymer of said film being selected from thegroup consisting of low density polyethylene, high density polyethylene,polypropylene, and polybutene.
 2. A method for limiting the increase insoil temperature in warm environments comprising mulching the soil withantistatic films obtained by the method consisting essentially ofcontinuously printing over at least part of the surface of at least oneside of a film of synthetic polymer with aluminum powder by suspendingin solvent a composition consisting essentially of aluminum powder and apolyamide resin, setting said suspension down onto said film, andpermitting said solvent to evaporate, said synthetic polymer of saidfilm being selected from the group consisting of low densitypolyethylene, high density polyethylene, polypropylene, and polybutene.